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Why are there two iPhone 6S A9 chips?

Apple has responded to claims that identical iPhone 6S and 6S Plus models can have very different battery performance depending on the origin of the A9 system on a chip (SoC). According to Apple, the power consumption of both chips varies within just two to three percent of each other -- but performance tests published online have caused controversy.

Unusually, Apple dual-sources its SoC packages from two different companies, each of which uses its own fabrication process to create the chips. It's here that differences creep in. An iPhone 6S will use either a 14nm Samsung APL0898, built on a 96mm2 die, or a 16nm TSMC APL1022, built on a 104.5mm2 die.

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Sites such as BGR have posted test results showing that TSMC's 16nm chip performs very slightly better than Samsung's and, more critically, that Samsung's smaller chip runs hotter and consumes more power. BGR claims that iPhones using the TSMC processor lasted around 50 minutes longer in battery tests than its Samsung equivalent.

However, the results are based on tests carried out on only a small number of handsets, making it hard to draw broad conclusions. Even BGR notes that "in regular day-to-day use, users aren't likely to tell the difference in performance".

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While it's obvious that the two companies use different fabrication methods, Apple claimed all its chips meet its performance standards. In a statement issued to TechCrunch the company further claimed that "lab tests which run the processors with a continuous heavy workload until the battery depletes are not representative of real-world usage, since they spend an unrealistic amount of time at the highest CPU performance state".

In the real world, based on customer data since the launch of the new iPhones, Apple says that the two different SoC packages -- and their power consumption -- "vary within just 2-3% of each other".

Regardless of by whom and how it's fabricated, the Apple A9 SoC includes a 64-bit 1.85 GHz ARMv8-A dual-core CPU called Twister, 2GB of LPDDR4 RAM, a dedicated image processor and an M9 motion coprocessor that handles data from the accelerometer, gyroscope and compass, as well as doing some of the voice recognition for Siri.

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As AnandTech points out, it's not entirely clear exactly why Apple would have two different companies fabricating the A9. The difference in fabrication processes used for each chip is down to active competition between TSMC and Samsung to be the first to get down to 14nm semiconductor device fabrication, a race that Samsung won with a chip that's 8 percent smaller in size than TSMC's 16nm SoC.

What's not clear from the limited data currently available from either Apple or third-party testers is how the 14nm Samsung chip's greater density actually impacts average power consumption across iPhone 6S in general, and whether or not that variation is, as Apple claims, bordering on statistically insignificant.

Updated 12/10/15, 10:08: The original version of this article contained an incorrect spelling of TSMC.